Nanobubbles Becoming the Subject of an Increasing Amount of Study

Despite initial scepticism, acceptance of the existence and special properties
of nanobubbles is now growing and their formation and characteristics are now
becoming the subject of an increasing amount of study, especially in Japan.
NanoSight
proves to be the method of choice for their study.

Due to the supposed very high pressure within bubbles of such small size and
radius of curvature and thus high surface tension, conventional calculations
show that the gas should be ‘pressed out’ of the nanobubbles within
microseconds. However, it is now clear that under the right conditions such
bubbles can both form freely and remain stable of extended time periods, sometimes
many months. Explanations as to just why such structures are so stable are focussing
on the role of counter-ions forming layers at the nanobubbles surface, which
helps explain claims that they apparently form only in the presence of salts.

Kaneo Chiba and Masayoshi Takahashi of Japan’s famous AIST research centre
have shown that in the presence of electrolytes and with the correct physical
stimulus, stable nanobubbles can be formed from conventional microbubbles. The
latter tend to either to coalesce to large buoyant bubbles which float away
or which collapse under intense surface tension-derived pressure to the point
they vanish as predicted by theory. The addition of salt (electrolytes) however,
is thought to cause the formation of a counter-ion screen around nanobubbles
which effectively blocks the ability of gases within the nanobubbles to diffuse
out. This was confirmed by electrophoresis studies in which the zeta potential
of nanobubbles was shown to be related to nanobubbles stability.

Furthermore, Professor William Drucker of the University of Melbourne has
also used infra-red spectroscopy to show that the pressure of gas within such
nanobubbles is not significantly higher than atmospheric, perhaps explaining
their stability and resistance to collapse.

There exists a wide range of proposed applications of nanobubbles and interest
in their usage is growing rapidly. When formed from ozone and electrolyte stabilised,
disinfection and sterilisation is possible for many months with great potential
in the preservation of foodstuffs and in medical applications as an attractive
alternative to chlorine based methodologies.

Oxygen nanobubbles have been implicated in the prevention of arteriosclerosis
by the inhibition of mRNA expression induced by cytokine stimulation in rat
aorta cell lines.

When formed in liquids in capillaries, nanobubbles have been shown to greatly
improve liquid flow characteristics. They have also been proposed as contrast
agents in scanning techniques as well as cleaning agents in silicon manufacturing
processes.

Finally, a new field of drug delivery applications is being actively researched
in which nanobubbles play an active part though details of this highly secretive
(and therefore important?) field are hard to come by. However, reported in Reuters,
Natalya Rapoport of the University of Utah's Department of Bioengineering is
using nanobubbles with the chemotherapy drug doxorubicin seek out cancer tumors
and congregate when injected into the bloodstream. "These nanobubbles don't
penetrate normal blood vessels but they do penetrate blood vessels in the tumor,"
said Rapoport, whose study appears in the Journal of the National Cancer Institute.
Once in the tumor, the nanobubbles combine to form larger "microbubbles,"
which can be seen on an ultrasound. "When these bubbles accumulate, I give
strong ultrasound radiation to the tumor to blow them up," she said in
a telephone interview. "Then the drug gets out of these bubbles locally
at the tumor site."

In mice, the nanobubbles were more effective at blocking tumor growth than
other nanoparticle delivery methods.

In a previous study, Kikuchi et al (2001) showed that hydrogen nanobubbles
formation was related to the influence of electrolysis conditions on the hydrogen
content and the diameter distribution of hydrogen nanobubbles. They analysed
the nanobubbles by Dynamic Light Scattering (DLS). However, in a recent study
of the formation and characterisation of nanobubbles in water by a major pharmaceutical
company in Japan, the concentration of nanobubbles in a mechanically formed
suspension of nanobubbles was found to be very low (<107/ml), a concentration
which would be too low for meaningful analysis of DLS. Of course, analysis by
Electron Microscopy was not possible because of the vacuum required for EM studies.
NanoSight was shown to be ideally suited to such analyses and in a blind experiment,
in which three samples of nanobubbles suspensions containing high, low and zero
numbers of nanobubbles were tested in duplicate, NanoSight results were found
to match exactly those predicted. The graph below shows the results in which
sample type A (Series 1 and 6 ) contained high concentration of nanobubbles,
sample B (Series 3 and 5) contained low concentration of nanobubbles and sample
C (Series 2 and 4) were control blanks. It should be noted that NanoSight allows
concentration of nanobubbles per unit volume to be estimated as well as size
and size distribution.

In a more recent project, Ichiro Otsuka (2008) of Ohu University, Japan has
studied the possible role of nanobubbles in ultra-high diluted samples of active
agents in which the phenomenon of succussion is considered to relevant. He used
NanoSight technology to examine nanobubbles formation and concentration in more
detail than was possible using an electrozone (Coulter) method or conventional
DLS.

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